If you believe you may have blocked the Duo Security phone number on your mobile phone you will need to determine which Duo Security number(s) were the number blocked and unblock it. It is possible that the number was not blocked by you but rather from your mobile provider (Verizon, AT&T, Sprint, etc). In this case, you can call your provider to see if the number can be whitelisted or try another method of authentication method (i.e. push or passcode).
The below information comes directly from the Duo Security site. More information can be found at https://help.duo.com/s/article/5516?language=en_US.
Phone call authentication for Duo can come from any of the numbers listed below. It's possible that numbers not on the list below may be marked as "Scam Likely" by some carriers.
Please note that these numbers may change over time: